Earth-boring tools for forming wellbores in subterranean earth formations generally include a plurality of cutting elements secured to a body. For example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”) include a plurality of cutting elements fixedly attached to a bit body of the drill bit. Roller cone earth-boring rotary drill bits may include cones mounted on bearing pins extending from legs of a bit body such that each cone is capable of rotating about the bearing pin on which it is mounted. A plurality of cutting elements may be mounted to each cone of the drill bit. In other words, earth-boring tools typically include a bit body to which cutting elements are attached.
The cutting elements used in such earth-boring tools often include polycrystalline diamond compacts (often referred to as “PDCs”), which act as cutting faces of a polycrystalline diamond material. Polycrystalline diamond material is material that includes inter-bonded particles in the form of grains or crystals of diamond material. In other words, polycrystalline diamond material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain,” “crystal,” and “particle” are used synonymously and interchangeably herein.
PDC cutting elements are conventionally formed by sintering and bonding together relatively small diamond grains under conditions of high temperature and high pressure in the presence of a catalyst (e.g., cobalt, iron, nickel, or alloys and mixtures thereof) to form a layer (referred to as a “compact” or “table”) of polycrystalline diamond (PCD) material on a cutting element substrate. These processes are often referred to as high-temperature/high-pressure (HTHP) processes. The cutting element substrate may comprise a cermet material (i.e., a ceramic-metal composite material) such as, for example, cobalt-cemented tungsten carbide. In such instances, the cobalt (or other catalyst material) in the cutting element substrate may liquefy and diffuse into the diamond grains during sintering and serve as a catalyst (which may also be characterized as a binder) for forming the inter-granular diamond-to-diamond bonds, and the resulting diamond table, from the diamond grains. In other methods, powdered catalyst material may be mixed with the diamond grains prior to sintering the grains together in an HTHP process.
Upon formation of a diamond table using an HTHP process, catalyst material may remain in interstitial spaces between the grains of diamond in the resulting PDC. The presence of the catalyst material in the diamond table may contribute to thermal damage in the diamond table when the cutting element is heated during use, due to friction at the contact point between the cutting element and the formation.
Polycrystalline diamond (PCD) typically contains more than 80 volume percent sintered diamond grains, with the balance a binder phase. As noted above, this binder phase is conventionally provided via infiltration from a supporting cemented carbide substrate, but may also be incorporated into the starting diamond powder as an admixture. The diamond grains typically lie within the 1 to 50 micron size range, but there is significant interest in incorporating a nanodiamond grain component, which may comprise a substantial portion, for increased PCD abrasion resistance, fracture toughness, and thermal stability. Such enhanced performance is believed to be attributable to increased diamond percent volume, augmented diamond particle interbonding, and reduction in catalyst material volume in the PCD.
However, significant problems have been encountered when diamond nanoparticles are incorporated in PCD. The diamond nanoparticles may dissolve in the liquid state, infiltrating binder from the substrate or admixed binder under HTHP process conditions, resulting in the loss of the beneficial characteristics of abrasion resistance, fracture toughness, and thermal stability provided by the presence of the diamond nanoparticles. Further, when a conventional cobalt-cemented tungsten carbide substrate is employed, sintering quality may be compromised due to the loss of binder volume available to the sintering process. This phenomenon becomes more evident when relatively higher concentrations of diamond nanoparticles, for example greater than about ten percent by volume, are employed to form PCD. In addition, the presence of tightly packed diamond nanoparticles in a volume of diamond grains undergoing HTHP processing inhibits infiltration of liquid-state binder from the substrate through the diamond table, causing a less well sintered region in the diamond table as distance increases from the interface between the diamond table and the substrate, resulting in poor abrasion resistance and compromised mechanical integrity of the diamond table.
The use of diamond-enhanced carbide (DEC) substrates has demonstrated improvements in creating more durable and impact resistant PDC cutting elements, as described in U.S. Patent Application Publication No. US 2011/0226532 A1, published Sep. 22, 2011, and titled “Insert for an Attack Tool, Method for Making Same and Tools Incorporating Same,” the disclosure of which is incorporated herein in its entirety by reference. Methods of forming DEC substrates are described in International Patent Publication WO 2009/128034 A1, published Oct. 22, 2009, and titled “Super-Hard Enhanced Hard-Metals,” the disclosure of which is incorporated herein in its entirety by reference.
The improvement in PDC cutting element durability and impact-resistance, as described in U.S. Patent Application Publication No. US 2011/0226532 A1, is due to the increase in Young's modulus of a “bolster” portion of the cutting element substrate comprising DEC due to the presence of diamond grains dispersed in a cobalt or other metal-cemented carbide substrate, as well as to size of the diamond grains employed. The substrate structure provides enhanced stiffness of support of the PCD table of the cutting element superior to that provided by a conventional, cobalt-cemented carbide substrate. However, an undesirable result of sintering a PCD table comprising micron-sized diamond grains using a DEC substrate is excessively high cobalt binder content in the as-formed PCD table and, consequently, a lower wear resistance may result.